1. Field of the Invention
This invention relates to a process for preparing a polyurethane elastomer. More particularly, it relates to a process for preparing a super-soft polyurethane elastomer having an improved thermal stability.
2. Description of the Prior Art
Polyurethane elastomers are used for various mechanical parts because of their excellent mechanical properties and their resistance to wear, solvent and weathering.
When a polyurethane elastomer is used as a vibration insulator, an impact absorber, or a medical plastic in particular, a super-soft property is required and its hardness should be extremely reduced.
According to the typical process, as described in the specification of British Pat. No. 1,564,195, super-soft polyurethane elastomer is prepared by reacting polyethylene glycol or polypropylene glycol with a polyisocyanate in less than stoichiometric amount in the presence of a tertially amine catalyst.
In this method, however, a polymer chain does not form completely in the resultant super-soft polyurethane elastomer.
The elastomer thus obtained can retain its shape and vibration absorbing ability at ordinary temperatures. And yet the elastomer greatly reduces load bearing capacity and is deformed or destroyed, when the temperature of elastomer is raised during use at elevated temperatures or as a result of heat generation due to the vibration. The medical plastic is required to stand disinfection by boiling.
A conventional organometallic polyurethane catalyst such as dibutyltin dilaurate or lead octoate is added as a countermeasure to accerelate polymerization. The catalyst, however, causes rapid viscosity increase of the ingredient mixture and makes molding procedure difficult. Furthermore, the permanence property of the super-soft polyurethane elastomer at elevated temperatures is reduced due to the deterioration effect of the catalyst.